1. Field of the Invention
The present invention relates to portable radiographic imaging apparatuses using solid-state imaging devices.
2. Description of the Related Art
To date, apparatuses for capturing radiographic images of objects by emitting rays to the objects and detecting the intensity distribution of the rays that pass through the objects are well-known and have been used in industrial non-destructive inspection and medical diagnosis. One common feature of such imaging methods is the use of a film/screen for detecting rays. Such a film/screen utilizes photosensitive films and fluorescent materials having sensitivity to rays in combination. Fluorescent sheets, which emit light when irradiated with rays, are adhered to either side of the photosensitive films, and the rays passing through the objects are converted into visible light in the fluorescent sheets. The light is received by the photosensitive films, and latent images formed on the photosensitive films are developed by chemical treatment so as to be visualized.
On the other hand, methods for capturing high-definition radiographic images have come into widespread use through recent progress in digital technology. In these methods, radiographic images are converted into electrical signals, and reproduced on, for example, cathode-ray tubes (CRTs) as visible images after image processing of the electrical signals. In order to convert radiographic images into electrical signals, systems for recording and reproducing radiographic images, in which images of transmitted rays are temporarily stored in fluorescent materials as latent images, and the latent images are photoelectrically reproduced by emitting exciting light such as laser beams so as to be output as visible images, have been proposed.
Moreover, apparatuses for capturing radiographic images in the same manner using semiconductor sensors have been developed through recent progress in semiconductor processing technology. These systems have markedly wide dynamic ranges compared with the known radiographic imaging systems using photosensitive films, and have practical advantages in capturing radiographic images that are substantially insusceptible to fluctuations in exposure to rays. At the same time, the systems do not require chemical processing in contrast to the known systems using photosensitive films, and have advantages in that they enable the instant obtainment of output images.
FIG. 9 is a conceptual view of a system using such a radiographic imaging apparatus. A radiographic imaging apparatus 2 accommodating a radiation-detecting sensor 1 for detecting rays is disposed in front of a subject S, and a radiation-generating apparatus 3 is disposed behind the subject S. Rays emitted from the radiation-generating apparatus 3 pass through the subject S, are converted into visible light via fluorescent materials (not shown) in the radiation-detecting sensor 1, and then are output as electrical signals by photoelectric transducers disposed in a two-dimensional grid pattern.
The radiographic apparatus 2 is connected to a controlling section 4 that controls reading of the electrical signals obtained in the radiation-detecting sensor 1, image transfer, and the like. The controlling section 4 performs digital image processing on the image signals output from the radiation-detecting sensor 1, and displays radiographic images of the subject S on a monitor 5.
Unlike the above-described systems for recording and reproducing radiographic images that read out images during postprocessing, this system can instantly display images on the monitor 5. Such imaging systems are placed on individual stands dedicated to capturing modes of, for example, an upright position and a supine position in a radiation room so as to be separately used as required.
In general, as is disclosed in Japanese Patent Laid-Open No. 2002-291730 shown in FIG. 10, such a radiographic imaging apparatus 2 has indicators 11a to 11c and indicators 12a to 12c for indicating an area that can receive rays for forming images, the indicators being marked on the top surface of the radiographic imaging apparatus 2. When the radiographic imaging apparatus 2 is mounted on a dedicated stand (not shown), parameters such as the central positions and the focal lengths of the radiation-generating apparatus 3 and the radiographic imaging apparatus 2 can be adjusted or measured in advance.
Furthermore, systems that can automatically position both of the apparatuses by receiving of capturing specifications are also known. Moreover, in recent years, thin and lightweight portable imaging apparatuses (electronic cassettes) available for capturing wide areas and for use when visiting patients as is disclosed in Japanese Patent No. 3577003 have also been developed. These apparatuses can be used in various capturing modes to which the above-described stationary imaging systems cannot be adapted.
However, although the above-described radiographic imaging apparatus 2 of a portable cassette type can be used in various capturing positions, alignment of the subject, an effective capturing area of the radiographic imaging apparatus 2, and an irradiation area of rays largely depends on the experience and skill of operators since the radiographic imaging apparatus 2 is not fixed on a dedicated stand and lacks positioning units or accurate measurement information for the alignment. In particular, when the radiographic imaging apparatus 2 is completely hidden under the subject, operators cannot confirm the outline of the radiographic imaging apparatus 2 and positioning becomes difficult.
In order to facilitate the alignment, a radiographic imaging apparatus having a large effective capturing area may be used for reducing the influence of the positional errors. However, this may cause increases in size and weight, and may in fact impair portability and operability of the apparatus.
Instant checking and recapturing of images are easy due to the characteristics of the radiographic imaging apparatus as a digital capturing system, but it is definitely not desirable to irradiate the subject with an unnecessary amount of rays.